The present invention relates to an optoelectronic display. Organic light emitting diodes (OLEDs) comprise certain organic materials which are known to emit light under electrical stimulation. The materials can either be small molecules or polymer materials (in polymer light emitting diodes, PLEDs). These materials require different processes for practical manufacture into display devices. Small molecule materials are deposited onto a substrate by vapour deposition whilst polymers are cast onto a substrate from a solution by spin-coating, printing, doctor blading or a reel to reel process. In a typical polymer LED, a polymer layer is deposited, by spin coating, onto indium tin oxide (ITO) coated glass. This is followed by heat treatment to drive off residual solvent and a reflective metal electrode is then evaporated onto the top surface of the polymer layer. The ITO, which is transparent, forms the other electrode and the polymer emits light through the ITO coated glass when a voltage is applied between the electrodes. Current and voltage control of the light emission is known.
Both types of materials and processes have been used to fabricate arrays on a number of different transparent and non-transparent surfaces. Methods known in the art for creating full colour displays include ink-jet printing of polymer solutions and vapour deposition of small molecule materials. Other known methods include the use of monochrome displays fitted with individual absorptive filters or colour changing media filters. Whilst both materials appear compatible with photo-resist technology, in practice the processing has reduced the efficiency and lifetime of the devices to unacceptable levels. High resolution colour and monochrome displays have been demonstrated for small molecules by depositing them into microcavities. In EP-0,774,787, a full colour OLED array is fabricated on a CMOS substrate by this method. The drivers for the diode array are formed in the substrate. The diodes are addressed by a passive matrix of conductive strips. For high resolution displays active matrix address schemes are preferable as they are more efficient.
Several different types of flat panel displays have been fabricated with active matrix address schemes. For instance, various types of liquid crystal display have been fabricated on crystalline silicon (LCOS) and other silicon materials such as polysilicon on glass. The silicon material provides the active matrix drive circuitry as well as the substrate. Similarly, a vacuum fluorescent display has been fabricated on crystalline silicon.
The manufacture of arrays of OLEDs on non-transparent substrates such as CMOS or bi-CMOS is hindered by the need to fabricate an (at least semi-) transparent electrode on top of the organic layers to allow light emission and viewing. Deposition of indium tin oxide directly onto the organic layers can cause unacceptable deterioration in the device performance. Another consideration is the need to carefully select the choice of metal electrode material directly in contact with the substrate so that it is fully compatible with microelectronic manufacturing equipment.